Dielectric ladder



Dec. 28, 1965 M. H. FINK DIELEGTRIG LADDER 2 Sheets-Sheet l Original Filed Aug. 21. 1961 b o 0 e 00u00 e e000 aa ooooooa INVENToge. www 7# FM BY MMM w Dec. 28, 1965 M. H. FINK DIELECTRIC' LADDER 2 Sheets-Sheet 2 Original Filed Aug. 21. 1961 INVENTOR. 7m@ 7% M BY//w /Ma United States Patent O 3,225,862 DIELECTRIC LADDER Miller H. Fink, Centralita, Mo., assignor to A. B. Chance Company, Centralia, M0., a corporation of Missouri Continuation of application Ser. No. 132,665, Aug. 21, 1961. This application Aug. 19, 1.963, Ser. No. 303,728 3 Claims. (Cl. 182-46) This is a continuation of my application Serial No. 132,665, filed August 2l, 1961, entitled Dielectric Ladder and Method of Constructing Same and now abandoned.

This invention relates to a dielectric ladder and t-he method for making same, and more particularly, it relates to a joint between the rail member and respective rung members of said ladder.

Ladders are often used around high tension lines to position a workman to reach certain components. Often the work is performed at a considerable elevation off the ground, thus making a relatively long ladder mandatory. Metallic ladders are strong, but they are subject to the disadvantage that they conduct electricity and thus increase the hazard to a lineman working from the ladder.

Ladders of dry, strong wood have relatively good dielectric properties and diminis-h the possibility of completing the circuit to ground through a linemans body and thence through the ladder. However, wooden ladders have a tendency to soak up moisture; moreover, their mechanical strength for a given weight is rather low.

It is therefore the object of the present invention to provide a ladder which will overcome the disadvantages of metal and wood in the prior art; more specifically, it is an object to provide such a ladder made of reinforced plastic with a moisture-resistant unicellular core and having rungs of a suitable dielectric material; it is a further object to provide simple and efficient joints between the rails and rungs on said ladder; and yet another object is to provide a method for making such a ladder and particularly for forming the joints for same.

In summary, the present invention concerns a ladder composed of spaced parallel rail members made of annular reinforced plastic casing or sleeve filled with a foam center and a multiplicity of dielectric rungs interconnecting said rail members. The joints between the rails and rungs depend on a bonded plastic collar which extends from the end of each rung into a socket-like interior in the rail. The collar is bonded to internal rail surfaces and it is of a larger diameter than the rung receiving opening which extends outwardly through the sides of the casing to receive and engage the rung end. The method of forming the joint contemplates that a hole will be drilled into the side of the rail and that an enlarged cavity will be dug out in the foam interior in communication with the exterior of the casing through the hole therein. The collar is formed in situ by filling the cavity with a curable liquid plastic material and curing said material to harden it while a rung is supported in place.

For a better understanding of the invention, reference may be made to the accompanying drawings wherein:

FIGURE 1 illustrates in elevation a ladder made in accordance with the present invention;

FIG. 2 is a transverse section, taken along line 2 2, through FIG. l',

FIG. 3 is a section taken along line 3 3 of FIG. 2;

FIG. 4 is a section taken along line 4 4 of FIG. 2; and

FIG. 5 is a transverse section illustrating the assembly of the rung to the rail.

Referring to FIG. l, 11 is a ladder embodying the present invention. It has elongated rails 13 and 15 interconnected by a multiplicity of tubular rungs 21.

Referring now to FIGS. 2-4, it will be seen that rail 13 has an outer casing or sleeve 17 and a central core ICC 19. The casing is preferably made of a reinforced plastic, e.g., a glass liber reinforced epoxy, and the core is preferably a unicellular plastic foam that is light and substantially impermeable to moisture, e.g., polyurethane. For a superior dielectric pole, the casing and the core should be bonded together to form a hermetic seal therebetween. My Patent 2,997,529 discloses the preferred materials and method of manufacture for an elongated electrical insulating rod, which I prefer to utilize yfor rails 13 and 15.

Rungs 21 are made of a strong material, preferably a dielectric. Various materials can be used, e.g., liber glass reinforced polyester rod, but I prefer that rungs 21 be made of the same materials as rails 13 and 15. Thus each rung 21 has a tubular portion 23 of glass reinforced plastic and an internal core 25 of unicellular plastic foam. The end portion 27 of rung 21 extends into the interior of rail 13 through the generally circular opening 29 in sleeve 17. The extremity of end 27 abuts the internal surfaces of casing 17 that lie opposite thereto, as indicated at 31 and 33 in FIG. 2. The foam core 25 of rung 21 terminates short of the outer extremity of rung end 27, thus providing exposed internal surfaces 35 of the reinforced plastic tubular portion 23.

Retainer collar 37 is bonded securely to the rung end 27 and extends radially outwardly therefrom to such a point that the over-all diameter of end 27 and collar 37 is significantly greater than the diameter of opening 29. This collar extends continuously from the inner wall surfaces 39 of casing 17 lying adjacent opening 29 to the inner wall surfaces 41 of casing 17 lying opposite opening 29, the casing being bonded tightly to these surfaces. The outer lateral surfaces 43 of collar 37 are configured generally as a right circular cylinder.

Foam core 19 encircles and jackets surfaces 43. Collar 37 extends into the recessed interior in the extremity of rung end 27 and in bonded engagement with the exposed internal rung surfaces 35 as well as with the exposed end surface of foam core 25.

It will be evident from the foregoing that rung 21 is securely retained from axial motion outwardly from rail 13 since collar 37 is larger than opening 29 and cannot pass therethrough and because of the variously described bonded connections between the rung end and the internal surfaces of rail casing 17. Moreover, the joint between rung 21 and rail 13 is so constituted that transverse loads applied to rung 21, acting in cantilever thereon, are resisted since the collar 37 integrally bonds the rung to the internal wall surfaces 39 adjacent opening 29 as well as t0 opposite wall surfaces 41. Thus, viewing the rung as a beam loaded transverse to its axis, it effectively has two points of support in its joint with rail 21: one at its end-most extremity, and the other displaced therefrom along the axis of rung 21 approximately the internal diameter of casing 17 Although the collar 37 is bonded securely with the jacketing foam surfaces in rail 13, and with the circular end surfaces of foam core 25 of rung 21, these joints cannot be depended on to add substantial strength to the over-all joint because of the structurally Weak nature of the unicellular foam, it being poor to resist a load in either tension or compression. So, while the foam-plastic collar bonds are excellent for sealing out moisture, they are of little assistance in adding joint strength.

The above described joint is formed in a relatively simple manner. Referring to FIG. 5, hole 29 is drilled through casing 17 and on through foam center 19, terminating against the internal wall surfaces 41 of the casing lying generally opposite hole 29. Then a suitable tool, for example, a drill with an expandable reamer means, is manipulated to remove additional foam and enlarge the passageway through the foam to define e11- larged cylindrical foam passage 51, which is of considerably greater diameter than hole 29. A knife or other instrument is used to scrape the foam from against the internal wall 41 opposite opening 29 and from the internal wall areas 39 just adjacent opening 29. The loosened particles of foam are removed from the internal rail cavity by inverting the rail and tapping it lightly or by blowing air into the opening with an air hose.

A recess in the end of rung 21 is prepared by removing foam to expose inner wall surfaces 35.

A measured quantity of liquid collar-forming material 53 is poured into cavity 51 while the rail is positioned with the opening 29 upright so that the collar-forming liquid 53 will not run out. The collar-forming liquid is a polymerizable plastic resin that will bond quite tightly with the exposed internal wall surfaces 39 and 41 of rail 13 and of the outer exposed surfaces of rung end 27 and with internal rung surfaces 35. Likewise, it should form a good moisture seal bond with the foam cavity defining surfaces 51 and the circular foam end of rung core 25. The material should be quite strong yet it should not be brittle. I ind that amine-cured epoxy resin which has been modied by an additive to make it somewhat more ilexible, and thus more resistant to crumbling, is quite suitable. One acceptable resin may be obtained by mixing two components commercially available from Shell Chemical Company under the designations Shell EPON 828 and Shell X-71 additive. Approximately 1:4 ratio of additive to resin, by weight, is preferable. An other commercially available resin is CIBAs Araldite No. 502. This resin requires no further additive. Both of the above resins cure at room temperature, and, depending on temperature and other ambient conditions, curing takes twenty minutes to an hour after addition of 6% to 8% by weight of an amine-curing agent, for example, diethylenetriamine.

Promptly after the liquid resin and curing agent, which are mixed just before pouring, are introduced into the cavity, rung 21 is placed into position by inserting rung end 27 into the cavity 51 and pushing its end-most portion into contact with the opposing inner wall surfaces 41 of rail casing 17. The positioned rung and rail are supported by a suitable jig until curing is substantially completed.

If an excess of liquid collar-forming material is used, it will be forced out through the slight clearance between hole 29 and rung 21 when the rung is inserted. This can be easily cleaned off by rubbing with a methyl ethyl ketone soaked rag.

Considering the assembly of a plurality of rungs, it will of course be appreciated that all the joints for at least one rail will have to be inserted substantially simultaneously. Since the resin takes several minutes before substantial curing occurs, there is ample time to ll each of the cavities with the liquid resin and position all of the rungs.

If desired, the ladder assembly may be accomplished by assembling the rungs to the rails, with all rung ends in place, and, while supporting the assembly by clamps or the like, introducing the collar-forming fluid into each of the joint cavities. This may be accomplished by drilling a small hole through the tubing of the rails to communicate with each of the internal cavities and introducing the liquid therethrough. This has the advantage of permitting that a seal be made between rung 21 and rail 13 beforehand in order that resin may not run out therebetween to require clean-up effort.

By way of example, a ladder of the instant invention may have side rails of approximately 21/2 inches diameter, the rungs being 1% inches in diameter. The diameter of the foam cavity in the rail at each joint would run say 1A to 3/8 of an inch greater than the diameter of the rung receiving opening in the outer tubular portion of the rail, which opening is just a bit larger than the diameter of a rung. For a 11A inches run, the hole would be about 1.26 inches. The quantity of resin and curing agent required is approximately 50 grams when the wall thickness of the outer tubing of the 21/2 inches rail is 0.155 inch, the wall thickness of the 1% inches rung 0.090, and the depth of the end cavity in the 1% inc-hes :rung 1/2 inch.

It will be appreciated that while my ladder is expected to find its greatest use as a safety ladder for work around high tension electricity, it will find some degree of application as a general-purpose ladder because of its light, strong construction.

Having thus described the invention what is claimed as new and desired to be secured by Letters Patent is:

1. A dielectric ladder assembly comprising:

a pair of spaced, generally parallel side rails; and

a plurality of spaced rungs connected to and spanning the distance between said side rails in generally prependicular relationship thereto, each of said rails and the rungs being defined by a tubular element of relatively rigid, noncellular, reinforced synthetic resin material and filled with a cellular, moisture-resistant, synthetic resin foam, said rails each being of greater diameter than the diameter of each rung and opposed portions of the rail elements having opposed, generally aligned openings therein for receiving the extremities of respective rungs, said openings being of only slightly greater diameter than said diameter of the rungs, there being a cavity of generally cylindrical configuration in the foam within each side rail aligned with respective openings in the elements thereof, coaxial with corresponding openings, of greater transverse diameter than respective openings and exposing the .rough interior surfaces of the rail elements in surrounding relationship to the openings as Well as indirect opposition to said openings, said extremities of the rungs extending through corresponding openings on the rail elements and terminating adjacent said exposed rough surfaces of the rail elements, a substantial portion of the extremities of the rungs positioned within said side rails being devoid of said foam and thereby defining inwardly extending depressions in each of the rungs communicating directly with corresponding cavities and exposing circumferentially extending internal surfaces of respective rung elements of major width, and a mass of solidifiedfmonolithic, noncellular, relatively rigid synthetic resin material filling each of said cavities and respective depressions aligned therewith, and rmly bonded to said exposed internal surfaces of the side rail elements and the rung elements to securely connect the rungs to the side rails and preclude rotation of the rungs relative to the side rails.

2. A ladder assembly as set forth in claim 1 wherein said side rail elements and the rung elements are of generally circular conguration in transverse cross-section.

3. A ladder assembly as set forth in claim 2 wherein said cavities and the depressions are of generally cylindrical configuration with adjacent cavities and depressions being in generally coaxial relationship.

References Cited by the Examiner UNITED STATES PATENTS 1,605,205 11/1926 Bellamy l56-293 2,804,322 8/1957 Herzog 156-293 2,862,650 12/1958 Scott 182-46 2,870,793 l/1959 Bailey. 3,009,532 11/1961 Richard et al. 182-46 3,042,140 7/ 1962 Basile et al. 182-46 HARRISON R. MOSELEY, Primary Examiner. REINALDO P. MACHADO, Examiner. 

1. A DIELECTRIC LADDER ASSEMBLY COMPRISING: A PAIR OF SPACED, GENERALLY PARALLEL SIDE RAILS; AND A PLURALITY OF SPACED RUNGS CONNECTED TO AND SPANNING THE DISTANCE BETWEEN SAID SIDE RAILS IN GENERALLY PREPENDICULAR RELATIONSHIP THERETO, EACH OF SAID RAILS AND THE RUNGS BEING DEFINED BY A TUBULAR ELEMENT OF RELATIVELY RIGID, NONCELLULAR, REINFORCED SYNTHETIC RESIN MATERIAL AND FILLED WITH A CELLULAR, MOISTURE-RESISTANT, SYNTHETIC RESIN FOAM, SAID RAILS EACH BEING OF GREATER DIAMETER THAN THE DIAMETER OF EACH RUNG AND OPPOSED PORTIONS OF THE RAIL ELEMENTS HAVING OPPOSED, GENERALLY ALIGNED OPENINGS THEREIN FOR RECEIVING THE EXTREMITIES OF RESPECTIVE RUNGS, SAID OPENINGS BEING OF ONLY SLIGHTLY GREATER DIAMETER THAN SAID DIAMETER OF THE RUNGS, THERE BEING A CAVITY OF GENERALLY CYLINDRICAL CONFIGURATIN IN THE FOAM WITHIN EACH SIDE RAIL ALIGNED WITH RESPECTIVE OPENINGS IN THE ELEMENTS THEREOF, COAXIAL WITH CORRESPONDING OPENINGS, OF GREATER TRANSVERSE DIAMETER THAN RESPECTIVE OPENINGS AND EXPOSING THE ROUGH INTERIOR SURFACES OF THE RAIL ELEMENTS IN SURROUNDING RELATINSHIP TO THE OPENINGS AS WELL AS IN DIRECT OPPOSITION TO SAID OPENINGS, SAID EXTREMITIES OF THE RUNGS EXTENDING THROUGH CORRESPONDING OPENINGS N THE RAIL ELEMENTS AND TERMINATING ADJACENT SAID EXPOSED ROUGH SURFACES OF THE RAIL ELEMENTS, A SUBSTANTIAL PORTION OF THE EXTREMITIES OF THE RUNGS POSITIONED WITHIN SAID SIDE RAILS BEING DEVOID OF SAID FOAM AND THEREBY DEFINING INWARDLY EXTENDING DEPRESSIONS IN EACH OF THE RUNGS COMMUNICATING DIRECTLY WITH CORRESPONDING CAVITIES AND EXPOSING CIRCUMFERENTIALLY EXTENDING INTERNAL SURFACES OF RESPECTIVE RUNG ELEMENTS OF MAJOR WIDTH, AND A MASS OF SOLIDIFIED, MONOLITHIC, NONCELLULAR, RELATIVELY RIGID SYNTHETIC RESIN MATERIAL FILLING EACH OF SAID CAVITIES AND RESPECTIVE DEPRESSIONS ALIGNED THEREWITH, AND FIRMLY BONDED TO SAID EXPOSED INTERNAL SURFACES OF THE SIDE RAIL ELEMENT AND THE RUNG ELEMENTS TO SECURELY CONNECT THE RUNGS TO THE SIDE RAILS AND PRECLUDE ROTATION OF THE RUNGS RELATIVE TO THE SIDE RAILS. 